Typically, you need to take at least one log backup on the principal database. However, a log backup might be unnecessary, if the database has just been created and no log backup has been taken yet, or if the recovery model has just been changed from SIMPLE to FULL.

Koen Verbeeck (12/11/2012)Are you sure a log backup is needed? Quoting from BOL:

Typically, you need to take at least one log backup on the principal database. However, a log backup might be unnecessary, if the database has just been created and no log backup has been taken yet, or if the recovery model has just been changed from SIMPLE to FULL.

I'm quite sure. The procedure in the question is the same I used to establish the mirror, I tried both wizard and T-SQL. The database was just changed from simple to full recovery model.I repeated the test when writing the QotD, with the same result. There was only one difference: First time the error message was completely confusing, but this was not reproduced.